Macrobenthos monitoring near the Sand Island and Barbers Point Ocean Outfalls, Oahu, Hawaii
Swartz, R.C., J.H. Bailey-Brock, W.J. Cooke, and E.A. Kay
The Sand Island and Barbers Point primary treatment plants discharge wastewaters into Mamala Bay through ocean outfalls located off the southern coast of O’ahu, Hawai’i at a water depth of approximately 60 m. The macrobenthos in the vicinity of these outfalls has been monitored since 1986. Replicate grabs at multiple stations located along transects and depth contours have been collected to provide biological samples for quantitative analyses. Samples for analysis of sediment grain size and parameters of sediment organic enrichment have also been collected. Benthic conditions at sites near the outfalls have been compared with reference conditions at sites 3 to 6 km from the outfalls to assess biological changes that might be related to the wastewater discharges. The structure of mollusk, nonmollusk (all macrobenthos excluding mollusks), and crustacean assemblages has been analyzed separately. Parameters of biological conditions include number of individuals, number of species, diversity (H1), evenness (J), faunal similarity (Bray-Curtis index), indicator species, and dominant species composition. Conditions have been compared in space (among stations from a single survey) and in time (among samples from different surveys; usually conducted near each outfall on an annual basis). Sediments, which were composed primarily of fine to coarse sands, showed no evidence of organic enrichment at any station. A diverse and abundant macrobenthos has been sampled near the two outfalls. Samples from each survey typically contained hundreds of benthic taxa and more than 10,000 individuals. Analyses of the abundance, taxa richness, and diversity of mollusks and nonmollusks almost always showed no statistically significant reductions at sites near the outfalls relative to reference sites. The abundance and taxa richness of crustaceans have often been less, but usually not significantly less, at some stations near the outfalls. The collection of a variety of pollution-sensitive amphipods near the outfalls indicates that the diminished crustacean fauna may be due to a noncontaminant factor. Separate cluster analyses of mollusks and nonmollusks indicate that factors associated with water depth are more closely related to faunal similarity than proximity to the outfalls. Long-term temporal trends show increased macrobenthic abundance. In summary, the weight of evidence indicates the Sand Island and Barbers Point discharges have not caused substantial changes in the macrobenthos.